The performance is exactly the same. The 7900GX2 is about 3 inches longer than the 7950GX2 but thats about the only difference. I dont think temps will vary much between the two either.
Architectually, the link between the 2 cards on the 7900GX2 runs at 16X while on the 7950GX2 its 8X. So in theory the 7900GX2 would be a little faster if you run it in a PCI express slot at full 16X. In the real world though the extra bandwidth makes no difference and they run are the same speed.
As for SLI issues, all games are different and depending on whether there is a working SLI profile existing for the game you play will depend on whether you get full performance.